The present invention relates to a linear phase ramp fiber optic gyro and, more particularly, to a closed loop type fiber optic gyro in which right-handed light and left-handed light having propagated through an optical fiber coil are caused by interference means to interfere with each other; a phase difference is provided between the right-handed light and the left-handed light by phase difference generating means disposed between the interference means and one end of an optical path; interference light available from the interference means is converted into an electrical signal; information about the phase difference between the right-handed light and the left-handed light is detected from the electrical signal; a ramp voltage of a frequency corresponding to the phase difference information is produced; and the ramp voltage is negatively fed back to the phase difference generating means so that the phase difference between the right-handed light and the left-handed light is reduced to zero.
FIG. 1 shows a conventional closed loop type linear phase ramp fiber optic gyro. Light emitted from a light source 11 is applied via an optical coupler 12 and a polarizer 13 to an optical coupler 14, from which it is incident, as right-handed light and left-handed light, to both ends of an optical fiber coil 15 which forms an optical path. The right-handed light and the left-handed light, which propagate through the optical fiber coil 15, are phase modulated by the output of an oscillator 17 in a phase modulator 16 disposed between one end of the optical fiber coil 15 and the optical coupler 14. The right-handed light and the left-handed light thus phase modulated are coupled together by the optical coupler 14 and interfere with each other, thereafter being applied again via the polarizer 13 and the optical coupler 12 to a photodetector 18 which serves as photoelectric conversion means. The interference light having thus reached the photodetector 18 is converted into an electrical signal, which is applied to a synchronous detector 19, wherein the same component as the phase modulation frequency is extracted from the signal. The output of the synchronous detector 19 is integrated by an integrator 21, and a ramp voltage of a frequency corresponding to the integrated output is generated by a ramp voltage generator 22. The ramp voltage is used to control a feedback phase generator 23 disposed as a phase difference generator between the optical coupler 14 and the other end of the optical fiber coil 15. The feedback phase generator 23 is formed by a phase modulator, which provides a phase difference between the right-handed light and the left-handed light and is controlled by the negative feedback thereto of the ramp voltage so that the phase difference between the right-handed and the left-handed light is reduced to zero.
The synchronous detector 19 detects information about the phase difference between the right-handed and the left-handed light. Letting the phase difference being represented by .DELTA..PHI., the output V of the synchronous detector 19 is as follows: EQU V=K.multidot.sin.DELTA..PHI. (1)
where K is a constant. The phase difference .DELTA..PHI. is expressed as follows: EQU .DELTA..PHI.=.DELTA..PHI..sub.106 +.DELTA..PHI..sub.f ( 2)
where .DELTA..PHI..sub..OMEGA. represents a Sagnac phase difference which results from the application of an angular velocity to the optical fiber coil 15 and is given by the following equation: ##EQU1## where R is the radius of the optical fiber coil 15, L is the length of the optical fiber coil 15, C is the velocity of light, .lambda. is the wavelength of light in a vacuum, .OMEGA. is an input angular velocity, and .DELTA..PHI..sub.f is the phase difference created by the feedback phase generator 23. The ramp voltage is applied from the ramp voltage generator 22 to the feedback phase generator 23, by which the right-handed light and the left-handed light undergo such phase shifts as indicated by the solid line (CW) and the broken line (CCW) in the upper portion of FIG. 2. The left-handed light is delayed behind the right-handed light by the time .tau. of propagation of light through the optical fiber coil 15. As a result of this, the phase difference .DELTA..PHI..sub.f between the right-handed and the left-handed light becomes such as shown in the lower portion of FIG. 2. If the feedback phase difference generator 23 is adapted so that a maximum value of the phase shift by the ramp voltage is 2.pi.k (where k is an integer), the phase difference .DELTA..PHI..sub.f between the right-handed and the left-handed light is given by the following equation: ##EQU2## where f is the frequency of the ramp voltage and n is the refractive index of the optical fiber coil 15. Since there is established a closed loop in which the feedback phase generator 23 is controlled so that the Sagnac phase difference .DELTA..PHI..sub..OMEGA. in the optical fiber coil 15 is cancelled, namely, the phase difference .DELTA..PHI. between the right handed and the left-handed light is reduced to zero, the frequency f of the ramp voltage is given, from Eqs. (2), (3) and (4), as follows: ##EQU3## By measuring the frequency f of the ramp voltage from Eq. (5), the input angular velocity .OMEGA. can be obtained using 2R/(n.multidot..lambda..multidot.k) as a proportional constant.
The output of this closed loop type fiber optic gyro is provided in the form of a pulse as indicated by Eq. (5). Incidentally, the pulse weight, i.e. an angular increment per pulse, can be obtained from Eq. (5) as follows: ##EQU4## In an actual medium-accuracy fiber optic gyro (L=300 m or so and R=0.020 or so) whose short-term bias stability is approximately 1.degree./hr (in the case where the sample time of angular velocity output is about 100 seconds), the pulse weight Pw is nearly equal to 6 arc-sec/pulse. In general, the value k is set as k=1.
In the case where the medium-accuracy fiber optic gyro whose pulse weight is nearly equal to 6 arc-sec/pulse or so as mentioned above is used to measure the attitude of a flying object by integrating pulses available from the ramp voltage, the influence of a quantization error (a 1-bit error) diminishes with the lapse of time and hence does not matter but poses a problem when accurate measurement of the angular velocity which is applied to the flying object is required.
For example, in the case of obtaining the applied angular velocity by sampling angular velocity output data every 0.01 second, an angular velocity noise, or a maximum error (quantization noise or 1-pulse noise) of 0.166.degree./sec.sup.p-p is generated depending on whether the last pulse is counted or not. This value of error is appreciably large.
Further, the pulse weight Pw is in inverse proportion to the radius R of the optical fiber coil 15 as indicated by Eq. (6). This is contradictory to an effort of improving the performance of optical and electrical systems of the fiber optic gyro to reduce the radius of the optical fiber coil to thereby attain miniaturization of the gyro. An increase in the pulse weight Pw caused an increase in the angular velocity noise, limiting the application to be made of this type of fiber optic gyro.